Electrical nerve stimulators have become widely used in recent years in the field of medicine for the treatment of chronic intractable pain. Such devices include circuitry for generating electrical pulses, and electrode leads for delivering the pulses to the site of the pain within the body. The electrical stimulating pulses produce the effect of masking the sensation of pain, and this method is preferable to drug therapy for many types of pain, because it avoids subjecting the patient to possible dangerous side effects. In the control of chronic pain by a nerve stimulator, there are generally provided adjustments or controls so that the stimulation delivered by the device can be adjusted or controlled according to the needs of the patient, which sometimes vary from day to day, or even minute to minute. Ideally, the pulse repetition rate, the pulse amplitude and the pulse width should be controllable to provide maximum flexibility in meeting the patient's needs.
Trancutaneous stimulators are worn or carried outside the body and have electrodes secured to the skin over the affected area to apply the electrical stimulation thereto. For some types of pain in certain locations of the body such as the spine or the brain, it is preferable to have an implantable lead with electrodes at the tip that can be positioned by the physician to the location of maximum effectiveness. For long-term treatment, it is preferable that the stimulating pulse output circuits also be implanted within the body so as to avoid the necessity of an electrical lead passing through the skin to external circuitry, since the site of a lead passing through the skin would have the potential for injury or infection.
An important type of implantable nerve stimulator is designed for use with an external controller-transmitter which provides not only the control of the repetition rate, pulse width and amplitude of the stimulating pulses, but also provides the energy for the pulses, transmitted electromagnetically by RF energy through the skin to the implanted unit. This avoids either having a lead extend through the skin, or having an implanted unit that depends upon batteries which have a finite life after which they must be replaced. The controller-transmitter unit has an antenna placed on the skin for close coupling with the antenna of the implanted unit so that energy will be transmitted thereto with reasonable efficiency.
Implantable nerve stimulators of this type have been provided in the prior art. In one prior art two channel device, the controller-transmitter unit consists of pulse generators for each of the channels, and a transmitter for sending bursts of RF energy to the antenna, under control of the pulse generators. The pulse generator for one channel causes transmission of a burst at a first RF frequency, for example 185 KHz, and the pulse generator for the second channel causes transmission of a burst at a second RF frequency, for example 460 KHz. The implanted unit has filter circuits tuned to these two transmission frequencies, and the outputs from the filters connect respectively to stimulating pulse output circuits for the two channels. Leads then connect from the output circuits to the stimulating sites within the body. For example, one lead may be positioned with its electrode along a location on the spine, while the other channel lead may extend to a stimulation site at the brain. A burst of RF energy causes delivery of a stimulating pulse to the channel corresponding to the frequency of the RF burst. Typically, pulse rates range from 10 to 100 pulses per second while pulse width varies between 0.05 and 2 milliseconds.
In this prior art device, the pulse width and the pulse amplitude for each channel can be independently adjusted by circuits in the controller-transmitter unit that control the amplitude and duration of the RF bursts, respectively. The frequency or repetition rate of the stimulation pulses can also be controlled by adjustment of the pulse generators in the controller-transmitter unit. However, repetition rates of the two channels cannot independently be adjusted, but instead they must operate at the same repetition rate, with their respective pulses or bursts out of phase with each other sufficiently so that the transmitter is not required to transmit both frequencies simultaneously. Transmission of both frequencies simultaneously would result in unwanted interaction both in the transmitter and in the implanted receiver section, resulting in undefined and undesirable pulses. Attempts to provide independent rate adjustments for the two channels would result in some of the pulses to the two channels coinciding or overlapping, leading to undesirable simultaneous transmission.
However, there is a need for independent rate adjustments of the multiple channels, since the nature of the pain being treated at the two different locations within the body often requires different rates that, in general, are totally independent of one another, and the prior art implantable nerve stimulator, described above, does not permit independent rates.